Answer: He conquered the kingdom of Lombard in northern Italy
The protection of the Pope.
Explanation: Charlemagne became the only ruler of Franks after the death of his Father and his brother. He started a military conquest where he conquered several areas like the Saxony in Germany and the Lombard in Northern Italy,his main aim was to extend Christianity to all the conquered territories and protect the papalcy.
He was finally installed as the Emperior of the Roman empire on Christmas day by Pope Leo III, in the year 800 AD.